The present invention relates to an improved process for producing aromatic hydroxycarboxylic acids.
Aromatic hydroxycarboxylic acids are important as antiseptics or preservatives for foods and cosmetics, or raw materials or intermediates of pigments, dyes, liquid crystals, liquid crystalline polymers, medicines, and agricultural chemicals and are generally produced by a solid-gas phase reaction known as the Kolbe-Schmitt reaction wherein an alkali metal salt of an aromatic compound having a phenolic hydroxyl group is reacted with carbon dioxide.
However, in the case where this reaction is conducted in a single step, there arises a problem concerning the formation of by-products. For example, in the case of salicylic acid synthesis, p-hydroxybenzoic acid and 4-hydroxy-isophthalic acid are by-produced in the total amount of about 0.4 percent by mole (see U.S. Pat. No. 4,376,867). Therefore, in order to obtain an aromatic hydroxycarboxylic acid at high purity, for example, a multi-step process is required because it is necessary to remove such by-products by incorporating to the reaction process a refining step such as sublimation.